Technical Field
The invention relates to a protection circuit for an inverter, in particular, for an inverter for an electric motor, with the inverter comprising a control system and a power component controlled by said control system, and with the control system being designed for the periodic output of a control pulse. Furthermore, the invention relates to a respective inverter system.
Related Art
Inverter systems are widely used in the field. An inverter system generates an alternating voltage from an input voltage, with its frequency and amplitude being selectable to a large extent. An important application of inverter systems is the control of electric motors, for example, permanently excited synchronous motors. Very frequently, the input voltage is provided by a power supply system, with the inverter system being connected to one or all three phases of said power supply system. In inverter systems with a direct current-link a rectifier generates a direct voltage from the input voltage; and this direct voltage is converted by a power component of the inverter system into the desired alternating voltage. For this purpose the power component comprises semiconductor switches, which are controlled by a control system, usually via pulse-width modulated signals (PMW signals). Frequently IGBTs (insulated gate bipolar transistors) or field effect transistors are used as semiconductor switches.
It may occur in inverter systems that individual semiconductor switches quit working correctly. In this case the faults, in which the semiconductor switch remains in a low impedance state, are relatively non-critical. In this case either the load current limiter engages or the semiconductor switch is destroyed as a result of the internal temperature rise. Also of relatively non-critical importance are conditions, in which the semiconductor switch remains in a sufficiently high-impedance state, because no problematic currents flowing into the load are expected. However, particularly critical are fault conditions, in which the semiconductor switch has such an impedance that the power loss at the semiconductor is too low to destroy the semiconductor due to an internal temperature rise and simultaneously preventing the d.c. link from short-circuiting. In this situation uncontrolled currents are conducted into the load, without here the usual protective measures taking effect, such as current limiting.
One way in which this state can develop is known as PTOM (partial turn on mode). In this situation an arbitrary control voltage can be applied to the semiconductor switch, without changing the state of the defective semiconductor switch. Thus, the control voltage can no longer control the semiconductor switch. A similar fault condition is the so-called loss-of-gate, in which the control voltage is no longer applied to the semiconductor switch, because, for example, a bonding wire is broken or a control system, which is supposed to emit the respective control voltage to the semiconductor switch, fails to function correctly. In this condition as well, the semiconductor switch remains in an undefined state.
In both fault situations an uncontrolled current is applied to the load; and said current can impermissibly heat this load or can trigger a malfunction or can even induce destruction of the load. When, for example, the inverter system generates a three-phase alternating current network for a synchronous motor and one leg of the inverter bridge no longer operates correctly, the rotating field generated in the stator is applied in an extremely unsymmetrical fashion. This leads to a considerable mechanic stress on the motor and can destroy the winding of the permanently energized coil of the stator. In both cases a safe state for the application and the user can no longer be ensured. A protection circuit is, therefore, necessary which also allows the inverter system to be safely shut off in the event of these fault conditions.
According to the fault analysis method of the VDE (Association of Electrical, Electronics, and Information Technology), it is also necessary to compensate for any other faults regarding the aforementioned PTOM and/or loss-of-gate. This primarily relates to a malfunction of the semiconductor control system, which under certain circumstances can no longer be shut off in a controlled fashion. In this case a protection circuit must also allow this combination of critical faults to be compensated for.